Electric machines are electromechanical energy converters that transform electrical energy into mechanical energy, mechanical energy into electrical energy, or both. Electric machines can include motors, generators, alternators, and rotary converters (motor-generators). These machines use a stator (a stationary part), and a rotor (a moving part), separated from one another by an air gap.
Various embodiments of electric machines include axial, rotary, and linear electric machines. In axial and rotary electric machines, the rotor has rotational motion. Rotary electric machines can be found in a “conventional” configuration, where the rotor is internal to the stator, or an “inside out” configuration, where the rotor is external to the stator. In linear electric machines, the moving part is still typically called a “rotor,” although the term is not used in its conventional sense as it does not rotate.
In both linear and rotary electric machines, the stator generally comprises a core and windings. A winding consists of coils of insulated wire or, in some cases, heavy, rigid insulated conductors. The winding may be placed around pole pieces, called salient poles, projecting into the air gap from one of the cores, or, with a stator core as shown in FIG. 1, the winding may be embedded in slots 14 cut into the core 12. In a slotted core, the core material remaining between the slots is in the form of teeth 28.
Both stator and rotor generally have a core of ferromagnetic material, such as silicon steel. The core is typically constructed using a bonded stack of thin ferromagnetic laminations that are electrically insulated from one another to impede the flow of eddy currents, which would otherwise greatly reduce the efficiency of the machine.
In all electric machines, electrical losses are an important factor in their design, and improved efficiency is desired. Furthermore, in high-performance systems, where current densities and time derivatives of magnetic flux density are high, heat flux is also high and special cooling means are required to prevent excessive temperatures within both the lamination stack and the winding. Many prior art designs trade off efficiency to improve thermal performance, or vice-versa.
One method used in the prior art to address the special cooling requirements discussed above includes the use of a housing, typically made out of aluminum, press-fitted to the stator core 12 and cooled by either external air flow over peripheral fins, or by the flow of liquid cooling fluid within the housing itself. A more advanced approach in the prior art, illustrated in FIG. 1, includes the use of cooling ports 10 within the stator core 12, allowing cooling fluid to be channeled directly into the core 12. In FIG. 1, the winding, which belongs within the slots 14, is omitted for clarity.
When high-performance cooling methods such as the above are employed, thermal performance is typically limited by the thermal resistance between the winding within the slots 14 and the core 12. Thermal resistance due to dielectric slot liners 18 and voids within the winding becomes the dominant element of the overall thermal impedance. Accordingly, as these components of thermal resistance are reduced, the overall thermal impedance is lowered and power levels can be further increased without incurring excessive temperatures.
Prior attempts to decrease these components of thermal resistance include the use of high thermal conductivity potting material 20 to fill the voids within the winding, as pictured in FIG. 2(a), and the use of large “bus” conductors 30 as the winding, in place of windings formed from multiple wire strands, pictured in FIG. 2(b).
Impregnating the winding as in FIG. 2(a) with a thermally conductive resin or potting material 20 provides a low resistance thermal path between the surface 22 of the individual conductors 24 of the winding and the core slot surfaces 26. While this method can reduce the thermal resistance, its effectiveness is limited by the finite thermal conductivity of available resins and poor resin penetration into the winding.
The use of large “bus” conductors 30 for the winding as in FIG. 2(b) is even more effective, because a greater portion of the slot 14 is filled with the metallic bus conductor 30, as opposed to potting material 20. This results in both reduced thermal resistance and reduced low frequency electrical resistance, both of which can allow increased winding currents without incurring excessive temperatures. Unfortunately, such bus windings 30 have several drawbacks, including increased cost, increased skin and proximity losses, and the requirement of larger slot openings, also resulting in increased electrical losses. Furthermore, because the cross-section of the teeth 28 is trapezoidal in shape to accommodate rectangular bus conductors 30, as opposed to teeth that are rectangular in shape as in FIGS. 1 and 2(a), the magnetic utilization of the teeth 28 is compromised. The net result is that the AC winding resistance is typically degraded, thus offsetting the above benefits.
Further information relevant to attempts to address these problems can be found in U.S. Pat. Nos. 2,711,008 (S. A. Smith); 4,745,314 (Nakano); 5,889,342 (Hasebe et al.); 6,710,479 (Yoshida et al.); 6,724,119 (Wellisch); 6,903,471 (Armitsu et al.); 6,954,010 (Rippel et al.); and 7,122,923 (Lafontaine et al). However, each one of these references suffers from one or more of the following disadvantages: limited thermal performance, increased cost, increased winding electrical resistance, or other increased losses in efficiency.
For the foregoing reasons, there is a need for an electric machine that can inexpensively improve thermal performance and efficiency.